Light from distant stars and galaxies travel through the near perfect vacuum of space for millions of years carrying the information about the distant parts of the universe until the last fraction of a second when it plunges through the earth's atmosphere before reaching our eyes or telescopes on the ground. Turbulence within the atmosphere causes degradation or aberration of images such that increasing the aperture of a telescope does not lead to improvement of resolution. Placing the telescope in space above the atmosphere or on top of mountains alleviates the problem but at considerable cost and constraint. Adaptive optics (AO) have been used over the past several decades to compensate for such aberration. When AO are used, the wavefront is sensed and adjusted by the optical system in real time to compensate for any distortion. Remarkable astronomical images have been obtained using ground-based telescopes with AO compensation of atmospheric turbulence, whose quality can even surpass space-based telescopes under favorable conditions. The principle of adaptive optics has also been applied to other imaging systems, most notably in ophthalmic imaging as well as in laser beam forming and remote sensing.
In a typical AO system, a guide star of sufficient brightness is used for measuring the aberration. A common method of wavefront sensing involves the use of a Shack-Hartmann sensor consisting of a lenslet array and a CCD camera underneath it. Distortions of the wavefront lead to shifting of lenslet focal spots proportional to the local slope of the wavefront. This shifting is used to compute the necessary displacement of a deformable mirror or other spatial light modulator. The wave-front sensing, wave-front modulation, and control subsystems form a closed loop to achieve a configuration that minimizes the aberration in the image of the full field.
While AO systems can greatly reduce distortion, they are not without their drawbacks. For example, AO systems are typically complex, high-precision systems that are both expensive and cumbersome. In addition, AO systems are also often slow to operate and prone to faults. It can therefore be appreciated that it would be desirable to have an alternative way to compensate for aberration.